Electrooptic modulator for frequency translation applications

ABSTRACT

A reflective modulator ( 10 ) includes a light splitter ( 12 ) which divides light into two portions, one of which is applied to a directional coupler ( 14 ) by a first path ( 20 ), and the other of which is applied to the directional coupler ( 14 ) by way of cascaded electrooptic (e-o) modulators ( 18   a   , 18   b ). One port of the directional coupler ( 14 ) is terminated in a reflector ( 22 ), and the other in an absorptive termination ( 24 ). Electrical signals A and B are applied to the modulators ( 18   a   , 18   b ), with the result of generation of sum and difference frequencies modulating the light. The light leaves the reflective modulator ( 10 ) and is coupled to a utilization apparatus ( 64 ) by a second directional coupler ( 28 ). A further e-o modulator ( 26 ) may be coupled in the first path, for controlling the long-term average phase shift. In a particularly advantageous embodiment of the invention, the signal sources ( 30   a   , 30   b ) are elemental antennas of an array ( 30 ).

This patent application is a divisional of allowed patent applicationSer. No. 09/173,264, filed Nov. 15, 1998 now U.S. Pat. No. 6,081,358.

FIELD OF THE INVENTION

This invention relates to communications, and more specifically toconversion between communications in optical form and electrical orelectromagnetic form.

BACKGROUND OF THE INVENTION

There has been increasing use of optical communications for terrestrialand vehicular use, partially because of the low cost and light weight ofmany optical components, particularly optical fiber transmission paths,and partially because an optical path can accommodate many light carrierwavelengths, each having a wide bandwidth capable of carrying multiplechannels of information. Optical paths are also resistant toelectromagnetic interference.

The wide bandwidth of optical channels allows carrying digitalinformation, and also allows carrying many channels of analoginformation, each on an individual subcarrier. This use, in turn,results in a need for simple and effective ways to shift each electricalbaseband frequency to a different carrier frequency. This frequencyshifting task is normally accomplished by a heterodyne mixer ormodulator.

Improved arrangements for frequency shifting baseband signals tosubcarrier frequencies are desired for use in optical communicationsystems.

SUMMARY OF THE INVENTION

An optical modulator according to an aspect of the invention includes asplitter/combiner light coupler including first, second, and thirdports. The splitter/combiner light coupler divides light applied to thethird port into substantially equal first and second portions at thefirst and second ports, respectively. A first optical directionalcoupler includes first and second coupled transmission paths, the firstcoupled transmission path of the first optical directional couplerdefines first and second ports, and the second coupled transmission pathof the first optical directional coupler defines first and second ports.A first light transmission path includes first and second cascadedelectrooptic portions. Each of the first and second electroopticportions includes an electrical input port, by which the index ofrefraction of each electrooptic portion is responsive to electricalsignals applied to its electrical input ports. The first lighttransmission path extends from the first port of the splitter/combinerlight coupler to the first port of the first transmission path of thefirst optical directional coupler. A second light transmission path isprovided, which extends from the second port of the splitter/combinerlight coupler to the first port of the second transmission path of thefirst optical directional coupler. An optical reflector is coupled tothe second port of one of the first and second transmission paths of thefirst optical directional coupler. An optical absorber is coupled to thesecond port of the other one of the first and second transmission pathsof the first optical directional coupler. As a result of thisarrangement, light applied to the third port of the splitter/combinerlight coupler is modulated by the (multiplicative) product of theelectrical signals applied to the electrical input ports of the firstand second electrooptic portions. In a particularly advantageous versionof this embodiment, a third electrooptic portion including an electricalinput port is coupled in cascade with the second light transmissionpath.

In order to separate the modulated light from the unmodulated light, areflective optical modulator as described above is further associatedwith a second optical directional coupler including a first port, asecond port, and a third port, for coupling light applied to the firstport to the second port, and for coupling light applied to the secondport to the third port, the second port of the second opticaldirectional coupler being coupled to the third port of thesplitter/combiner light coupler, whereby signals applied to the firstport of the second optical directional coupler are coupled to thereflective modulator, and modulated light from the reflective modulatoris coupled to the third port of the second optical directional couplerby way of the second port of the second optical directional coupler.

Another hypostasis of the invention is an array antenna, which includesa plurality of antenna elements, each including a port at which receivedsignals appear, and also includes an optical fiber onto which lightsignals representing the individual signals received by the plurality ofantenna elements are to be coupled. A source of light is provided, as isa source of a plurality of electrical carriers, each at a frequencywhich is offset from the frequencies of the other carriers. A pluralityof optical modulators is included, each including a light port, andfirst and second electrical ports. Each of the optical modulatorsmodulates light applied to its light port in response to the product ofthe electrical signals applied to the first and second electrical ports.Each one of the plurality of optical modulators has its first electricalport coupled to the port of one of the plurality of antenna elements,its second electrical port coupled to the source of a plurality ofelectrical carriers, for receiving one of the carriers therefrom, andits light port coupled to the source of light. A coupling arrangement iscoupled to the optical fiber and to the light ports of the plurality ofoptical modulators.

In a particular avatar of the array antenna according to the invention,each optical modulator comprises a splitter/combiner light couplerincluding first and second ports, and the light port. Thesplitter/combiner light coupler divides light applied to the light portinto (preferably equal) first and second portions at the first andsecond ports, respectively. A first optical directional coupler includesfirst and second mutually coupled transmission paths. The firsttransmission path of the first optical directional coupler defines firstand second ports, and the second transmission path of the first opticaldirectional coupler defines first and second ports. A first lighttransmission path includes first and second cascaded electroopticportions. The first and second electrooptic portions include the firstand second electrical input ports, respectively. The index of refractionof the electrooptic portions is responsive to electrical signals appliedto the electrical input ports. The first light transmission path extendsfrom the first port of the splitter/combiner light coupler to the firstport of the first transmission path of the first optical directionalcoupler. A second light transmission path extends from the second portof the splitter/combiner light coupler to the first port of the secondtransmission path of the first optical directional coupler. An opticalreflector is coupled to the second port of one of the first and secondtransmission paths of the first optical directional coupler. An opticalabsorber is coupled to the second port of the other one of the first andsecond transmission paths of the first optical directional coupler. As aresult, the light applied from the source of light to the light port ofthe splitter/combiner light coupler is modulated by the multiplicativeproduct of the electrical signals applied to the electrical input portsof the first and second electrooptic portions. In a particularmanifestation of this avatar, the coupling arrangement comprises asecond light directional coupler associated with each of thesplitter/combiner light couplers. Each of the second light directionalcouplers includes a first port, a second port, and a third port, forcoupling light applied to the first port to the second port, and forcoupling light applied to the second port to the third port. The secondport of the second optical directional coupler is coupled to the thirdport of the associated one of the splitter/combiner light coupler. As aresult, signals applied to the first port of the second opticaldirectional coupler are coupled to the optical modulator, and modulatedlight from the optical modulator is coupled to the third port of thesecond optical directional coupler by way of the second port of thesecond optical directional coupler.

A method for receiving signals by way of an array antenna includes thesteps of: (a) generating carrier light; (b) generating a plurality ofelectrical carrier signals, each of which is offset in frequency fromall of the other electrical carrier signals, to thereby produce offsetcarrier signals; (c) applying carrier light to a light splitter, fordividing the carrier light into at least first and second portions,preferably of equal amplitude; (d) transmitting the first portion of thecarrier light to a first port of a four-port directional coupler by wayof a first light path; (e) transmitting the second portion of thecarrier light to a cascade of first and second electrooptic modulators;(f) applying received signal from one of the antenna elements of thearray antenna to one of the first and second electrooptic modulators,for modulating the second portion of the light; (g) coupling one of theplurality of electrical carrier signals to the other one of theelectrooptic modulators, for further modulating the second portion ofthe light; (h) coupling modulated light from the cascade of first andsecond electrooptic modulators to a second port of the four-portdirectional coupler; (i) reflecting light exiting from the third port ofthe four-port directional coupler, so that the reflected light re-entersthe third port and flows, at least in part, to the fourth port of thefour-port directional coupler, where the reflected light sums with lightcoupled from at least one of the first and second ports; (j) absorbingany net light leaving the fourth port, whereby the interaction of thelight reaching the fourth port variously results in reinforcement(addition) and cancellation (subtraction), which results in mixing ofthe carrier light, and received and electrical carrier signals, toproduce carrier light mixed with the received electrical signals and theone of the offset carrier signals; (k) for each pair of the receivedsignals and electrical carrier signals, repeating the steps of applyingcarrier light to a light splitter, transmitting the first portion,transmitting the second portion, applying received signal, coupling oneof the plurality of electrical carrier signals, coupling modulatedlight, reflecting light, and absorbing any net light; (l) combining theresulting carrier light mixed with the received electrical signals andoffset carrier signals, to produce combined signals. The resultingoptical signals representing the received electrical signal at eachantenna element can be transmitted over the same optical path, such asan optical fiber path, since the signals are at different opticalwavelengths.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a simplified diagram in block and schematic form, illustratingan arrangement by which electrical signals from a plurality of sources,which may be elements of an antenna, may be coupled over an opticaltransmission path;

FIG. 2 is a simplified diagram, in block and schematic form, of anelectrooptical modulator for frequency translation, according to anaspect of the invention;

FIG. 3 plots reflected light intensity of a portion of the arrangementof FIG. 2 in response to relative phase in a leg of the arrangement ofFIG. 2;

FIG. 4 illustrates another embodiment of the arrangement of FIG. 2, inwhich long-term average phase can be controlled; and

FIG. 5 is a simplified diagram in block and schematic form, illustratinghow the arrangement of FIG. 3 can be adjusted for optimum operation.

DESCRIPTION OF THE INVENTION

In FIG. 1, a set 50 includes a plurality of sources of electricalsignal, which are illustrated as blocks 50 a, . . . , 50N. In acommunication system, these may be discrete sources of different data,or in the context of an array antenna, they may represent the variouselemental antennas (antenna elements) of the array. The electricalsignals produced by the source set 50 are generally termed “radiofrequency” (RF) signals. The term “RF” at one time referred toelectrical signals in the frequency range from 550 to 1600 KHz, but morerecent usage applies the term to any frequency from above the audiorange of about 20 or 30 KHz to the lower infrared frequencies. Theelectrical signals produced by the set S0 are individually applied toinput ports 52 a ₁, . . . , 52N₁ of an array 52 of multipliers ormodulators 52 a, . . . , 52N. Multipliers 52 a, . . . , 52N of set 50also each receive an electrical carrier signal at second input ports 52a ₂, . . . , 52N₂ from a block 54, which is designated as a “comb” orechinate signal generator. A comb signal is a set of electrical signalshaving equal frequency increments, and often connotes equal amplitudes.In the arrangement of FIG. 1, the comb signals may be viewed as being atfrequencies of F, F+n, F+2n, . . . , F+Nn, where n is a frequencyincrement, and N corresponds to the number of RF sources of set 50. Inthe arrangement of FIG. 1, frequency F is applied over signal path 55 ato second input port 52 a ₂ of multiplier 52 a, and frequency F(N−1)n isapplied over signal path 55N to second input port 52N₂ of multiplier52N. Multipliers 52 a, . . . , 52N of set 50 multiply the two signalsapplied to their input ports, and produce sum and difference signals,each of which is offset from the next by the offset frequency n. Theoffset signals from set 50 of multipliers are applied through a set ormultiplexer 56 of bandpass filters, each of which passes or selects oneof the sum or difference frequencies, and rejects the other. Thus, theRF signal from source 50 a is multiplied by frequency F in multiplier 52a, and the resulting sum and difference electrical signals are appliedto bandpass filter (BPF) 58 a, which passes the sum signal, and rejectsthe difference. Similarly, the RF signal from source 50N is multipliedby frequency F(N−1)n in multiplier 52N, and the resulting sum anddifference signals are applied to BPF 58N, which passes the sum signal,and rejects the difference.

The sum electrical signals passed by the bandpass filters 58 a, . . . ,58N of multiplexer set 56 of bandpass filters of FIG. 1 are appliedjointly (together) to an RF-to-light converter illustrated as a block60, which converts the electrical signals into a modulated light signal.Such RF-to-light converters are very well known, and in their simplestform amount to no more than a solid-state laser. More particularly, thelight signal is in the form of a wavelength of light, modulated by thelight equivalent of the individual RF signals, each at a wavelengthoffset from others of the light-equivalent RF signals by a wavelengthcorresponding to the offset frequency n. The modulated light signal isapplied from converter 60 to a light transmission path illustrated as anoptical fiber 62, but which may include free-space light transmissionpaths, wavelength conversion, amplification, branching, and the like.The light transmission path 62 ends at a light-to-RF converterillustrated as a block 64. Such light-to-RF converters are also wellknown, and may be as simple as a photodiode. Light-to-RF converter 64converts the light arriving from signal path 62 into RF, which, inprinciple, is identical to that which was applied to RF-to-lightconverter 60. Thus, the RF signal at the output of light-to-RF converter64 includes the comb of offset frequencies, each modulated by theassociated RF signal from one of the sources of set 50. A demultiplexer66 separates the individual signals into different paths 67 a, . . . ,67N, from which the individual signals are applied to RF detectors 68 a,. . . , 68N. Detectors 68 a, . . . , 68N demodulate the signals torecover the information carried thereon, which is coupled to signalpaths 70 a, . . . , 70N.

It will be noticed that conversion of the array of RF signals fromsource set 50 in the arrangement of FIG. 1 requires RF multipliers 52 a,. . . , 52N of set 52, multiplexer 56, and RF-to-light converter 60. Inthe context of an array antenna, the presence of such RF processingequipment is often inconvenient, because of its bulk.

In FIG. 2, a light source 34 generates light, and applies it to an inputport 28 ₁ of a light directional coupler 28 of an optical modulator 11.The light from source 34 propagates through coupler 28, and exits at aport 28 ₂. The light exiting port 28 ₂ of coupler 28 is applied to areflective light modulator 10, and more particularly to an input/outputport 12 ₃ of a splitter/combiner 12 of reflective modulator 10. Lightapplied to port 12 ₃ of splitter/combiner 12 is split into two portions,ideally of equal amplitude, and appears at ports 12 ₁ and 12 ₂.

In FIG. 2, a light directional coupler 14 of reflective modulator 10includes two mutually coupled transmission paths 14 a and 14 b. Coupledpath 14 a has two ports, 14 ₁ and 14 ₂. Coupled path 14 b has two ports,14 ₃ and 14 ₄. The light which leaves port 12 ₂ of splitter/combiner 12is coupled by way of an optical fiber path illustrated as 20 to port 14₃ of the first transmission path 14 a of mutually coupled transmissionpaths 14 a, 14 b. The light which leaves port 12 ₁ of splitter/combiner12 flows to port 14 ₁ of directional coupler 14 by way of a path 16.Path 16 includes a cascade of first and second electrooptic modulatorsor phase shifters designated 18 a and 18 b. While illustrated anddesignated separately, electrooptic modulators or phase shifters 18 a,18 b may be part of the same structure, requiring only separatemodulation functions. Such electrooptic phase shifters are well known,and require only imposition of an electric field across electrodes on anelectrooptic material through which the light flows. As illustrated inFIG. 2, a source of RF 30 a has its electrical output port 30 _(ap)coupled to an electrical input port 18 ae of modulator 18 a, and asource of RF 30 b has its electrical output port 30 _(bp) coupled toelectrical input port 18 be of modulator 18 b. A light-reflectivetermination 22 is coupled to port 14 ₄ of directional coupler 14, and alight-absorbing termination 24 is coupled to port 14 ₂ of directionalcoupler 14.

In operation of the modulator arrangement 10 of FIG. 2, light applied toinput port 12 ₃ of splitter/combiner 12 is split into two portions, afirst portion flows over optical path 20 to directional coupler 14, anda second portion flows over optical path 16, including modulators 18, todirectional coupler 14. Directional coupler 14 couples a portion of thelight applied to its port 14 ₁ directly through path 14 a to port 14 ₂,and another portion to port 14 ₄. Also, directional coupler 14 couples aportion of the light applied to its port 14 ₃ directly through path 14 bto port 14 ₄, and another portion to port 14 ₄. Thus, a portion of thelight applied to each of ports 14 ₁ and port 14 ₂ appears at each ofports 14 ₂ and 14 ₄. Since an optical reflector or mirror 22 is coupledto port 14 ₄, all the light which exits port 14 ₄ is reflected back tothe port, and is coupled to ports 14 ₁ and 14 ₃. Any net light whichexits port 14 ₂ of directional coupler 14 is absorbed by absorbenttermination 24.

As mentioned, it is the net light which exits port 14 ₂ of directionalcoupler 14 which is absorbed by absorbent termination 24. Since thelight exiting port 14 ₂ includes components from paths 16 and 20, theirphases may differ by the amount of any inherent phase shift between thepaths 16 and 20, plus any phase shift imparted by the modulators 18 a,18 b in response to RF signals from sources 30 a and 30 b. Under somecircumstances, the two components of the light appearing at port 14 ₂ ofdirectional coupler 14 will be in-phase, and the components will sum,and be absorbed by absorbent termination 24. Under other conditions, thetwo components of light appearing at port 14 ₂ of directional coupler 14will be out-of-phase, and the components will cancel or partiallycancel. When two light signals cancel, the result is no light. One mightview the effect of cancellation of the two signal components at port 14₂ as turning absorbent termination 24 into a reflective termination. Putanother way, changing the phase of the signal traversing path 16 ofreflective modulator 10 of FIG. 2 over its full range results invariation of the amount of reflection at port 14 ₂. It should be notedthat the same cancellation and reinforcement of the light componentsoccurs at port 14 ₄, but is irrelevant since the result is alwaysreflection. FIG. 3 plots the reflection of light as measured at port 12₃ of splitter/combiner 12 of modulator 12, as a function of total phaseΘ_(A)+Θ_(B) contributed by the phase modulator sections 18 a and 18 b.As illustrated in FIG. 3, the plot 310 ranges from an amplitude of zeroto a maximum amplitude, which may be interpreted as total reflection,except for unavoidable losses in the optical components. Near each ofthe peak values and minimum values of plot 310 is a region designated312, which may be termed “parabolic,” in which there is an approximatelyquadratic relationship between the phase and the light intensity. Such arelationship of the intensity I to the magnitudes A and B of theelectrical signals may be expressed as

I=(A+B)²  1

in which case the frequency components of A and B will mix, in such amanner as to produce sum and difference frequencies.

Thus, the light reflected by the reflective modulator 10 of FIG. 2includes sum and difference frequencies of the electrical components Aand B, so long as operation is maintained in the parabolic regions 312of FIG. 3. Reflection of light by reflective modulator 10 of FIG. 2results in light leaving reflective modulator 10, and proceeding intoport 28 ₂ of directional coupler 28. The directional coupler routes thereflected light from its port 28 ₂ to port 28 ₃, whence the reflectedlight, with its modulation, is separated from the incident light appliedfrom source 34 to coupler 28, and the reflected light becomes availableon optical path 32. Modulator 11 of FIG. 2 can be viewed as being ablock having two electrical input ports, namely 18 ae and 18 be, a lightinput port 28 ₁, and a light output port 28 ₃.

FIG. 4 illustrates an arrangement for performing some of the functionsof FIG. 1 using the modulator 11 of FIG. 2. In FIG. 4, a light source 34is coupled to a light splitter 80, which divides the light into Nportions, which are applied over optical paths 80 ₁, . . . , 80N to theinput ports 28 ₁ of a plurality of optical modulators, each identical tomodulator 11 of FIG. 2, which are designated 11 a, . . . , 11N. Eachoptical modulator 11 a, . . . , 11N of FIG. 4 has four ports, namely alight input port 28 ₁, first and second electrical input ports 18 ae and18 be, and a modulated light output port 28 ₃, all corresponding to theports of modulator 11 of FIG. 2 taken as a block.

In the arrangement of FIG. 4, each optical modulator 11 a, . . . , 11Nreceives, at its input port 18 ae, an electrical RF signal from theoutput port 50 ap, . . . , 50Np of an associated source 50 a, . . . ,50N, respectively, and an RF offset frequency from an output of combsignal generator 36 at its input port 18 be. Each modulator 11 a, . . ., 11N performs modulation as described in conjunction with FIGS. 2 and3, and the resulting modulated light is output from its modulated lightoutput port 28 ₃ onto a signal path 32 a, . . . , 32N. The modulatedlight signals are collected by a coupler 38, and coupled onto opticalpath 62. Coupler 38 does not have to be a wavelength-dependentmultiplexer, because the light signals appearing on signal paths 32 a, .. . , 32N are already at disparate frequencies or wavelengths, as aresult of the modulation. The arrangement of FIG. 4 is advantageous bycomparison with the arrangement of FIG. 1 because the modulationrequires no RF bandpass filters, multiplexers or multipliers. Instead,all of the modulation can be performed directly in an opticalarrangement, which is more suitable for placement near the elements 50a, . . . , 50N of an array antenna.

FIG. 5 is a simplified diagram, in block and schematic form, of anarrangement according to an aspect of the invention, in which areflective modulator 10 as described in conjunction with FIG. 2 has anadjustment for long-term average phase. Elements of FIG. 5 correspondingto those of FIG. 2 are designated by like reference numerals. In FIG. 5,path 20 includes a further electrooptic element 26, the electrode 26 aof which is coupled by a conductive path designated 26 ae to a source ofcontrollable electrical bias, represented by a battery arrow symbol 28.In operation, the bias provided by source 28 is adjusted to optimizeoperation in the square-law region, represented by portions 312 of theplot of FIG. 3.

Other embodiments of the invention will be apparent to those skilled inthe art. For example, the bias provided by source 28 may beautomatically adjusted by a simple controller which senses the amplitudeof either or both of the sum and difference signals produced at port 28₃ of coupler 28, and perform a continuing process of iterating the biasthrough a peak amplitude of the sum/difference signal, past the peak bya small amount, and reversing control direction to again pass throughthe peak. Such a controller is described generally, in a different, andmore complex context, in U.S. Pat. No. 4,882,547, issued Nov. 21, 1989in the name of Katz.

Thus, an optical modulator (10) according to an aspect of the inventionincludes a splitter/combiner light coupler (12) including first (12 ₁),second (12 ₂), and third (12 ₃) ports. The splitter/combiner lightcoupler (12) divides light applied to the third port (12 ₃) intosubstantially equal first and second portions at the first (12 ₁) and(12 ₂) second ports, respectively. A first optical directional coupler(14) includes first (14 a) and second (14 b) coupled transmission paths,the first coupled transmission path (14 a) of the first opticaldirectional coupler (14) defines first (14 ₁) and second (14 ₂) ports,and the second coupled transmission path (14 ₂) of the first opticaldirectional coupler (14) defines first (14 ₃) and second (14 ₄) ports. Afirst light transmission path (16) includes first (18 a) and second (18b) cascaded electrooptic portions. Each of the first (18 a) and second(18 b) electrooptic portions includes an electrical input port (18 ae,18 be), by which the index of refraction of each electrooptic portion(18 a, 18 b) is responsive to electrical signals applied to itselectrical input ports (18 ae, 18 be). The first light transmission path(16) extends from the first port (12 ₁) of the splitter/combiner lightcoupler (12) to the first port (14 ₁) of the first transmission path (14a) of the first optical directional coupler (14). A second lighttransmission path (20) is provided, which extends from the second port(12 ₂) of the splitter/combiner light coupler (12) to the first port (14₃) of the second transmission path (14 b) of the first opticaldirectional coupler (14). An optical reflector (22) is coupled to thesecond port (14 ₂; 14 ₄) of one of the first (14 a) and second (14 b)transmission paths of the first optical directional coupler (14). Anoptical absorber (24) is coupled to the second port (14 ₂; 14 ₄) of theother one of the first (14 a) and second (14 b) transmission paths ofthe first optical directional coupler (14). As a result of thisarrangement, light applied to the third port (12 ₃) of thesplitter/combiner light coupler (12) is modulated by the(multiplicative) product of the electrical signals applied to theelectrical input ports (18 ae, 18 be) of the first (18 a) and second (18b) electrooptic portions. In a particularly advantageous version of thisembodiment, a third electrooptic portion (26) including an electricalinput port (26 e) is coupled in cascade with the second lighttransmission path (20).

In order to separate the modulated light from the unmodulated light, anoptical modulator (10) as described above is further associated with asecond optical directional coupler (28) including a first port (28 ₁), asecond port (28 ₂), and a third port (28 ₃), for coupling light appliedto the first port (28 ₁) to the second port (28 ₂), and for couplinglight applied to the second port (28 ₂) to the third port (28 ₃), thesecond port (28 ₂) of the second optical directional coupler (28) beingcoupled to the third port (12 ₃) of the splitter/combiner light coupler(12), whereby signals applied to the first port (28 ₁) of the secondoptical directional coupler (28) are coupled to the optical modulator(10), and modulated light from the optical modulator (10) is coupled tothe third port (12 ₃) of the second optical directional coupler (28) byway of the second port (28 ₂) of the second optical directional coupler(28).

Another hypostasis of the invention is an array antenna, which includesa plurality of antenna elements (30 a, 30 b, 30N), each including a port(30 ap, 30 bp) at which received signals appear, and also includes anoptical fiber (32) onto which light signals representing the individualsignals received by the plurality of antenna elements (30 a, 30 b, 30N)are to be coupled. A source of light (34) is provided, as is a source(36) of a plurality of electrical carriers, each at a frequency which isoffset from the frequencies of the other carriers. A plurality ofoptical modulators (10) is included, each including a light port (12 ₃;28 ₁), and first (18 ae) and second (18 be) electrical ports. Each ofthe optical modulators (10) modulates light applied to the light port(12 ₃) in response to the product of the electrical signals applied tothe first (18 ae) and second (18 be) electrical ports, which are inputports. Each one of the plurality of optical modulators (10) has itsfirst electrical port (18 ae) coupled to the port (30 ap, 30 bp) of oneof the plurality of antenna elements (30 a, 30 b, 30N), its secondelectrical input port (18 be) coupled to the source (36) of a pluralityof electrical carriers, for receiving one of the carriers therefrom, andits light port (12 ₃; 28 ₁) coupled to the source of light (34). Acoupling arrangement ((28, 38) is coupled to the optical fiber (32, 32a, . . . , 32N) and to the light ports (12 ₃) of the plurality ofoptical modulators (10).

In a particular avatar of the array antenna according to the invention,each optical modulator (10) comprises a splitter/combiner light coupler(12) including first (12 ₁) and second (12 ₂) ports, and the light port(12 ₃). The splitter/combiner light coupler (12) divides light appliedto the light port (12 ₃) into substantially equal first and secondportions at the first (12 ₁) and second (12 ₂) ports, respectively. Afirst optical directional coupler (14) includes first (14 a) and second(14 b) mutually coupled transmission paths. The first transmission path(14 a) of the first optical directional coupler (14) defines first (14₁) and second (14 ₂) ports, and the second transmission path (14 b) ofthe first optical directional coupler (14) defines first (14 ₃) andsecond (14 ₄) ports. A first light transmission path (16) includes first(18 a) and second (18 b) cascaded electrooptic portions. The first (18a) and second (18 b) electrooptic portions including the first (18 ae)and second (18 be) electrical input ports, respectively. The index ofrefraction of the electrooptic portions (18 a, 18 b) is responsive toelectrical signals applied to the electrical input ports. The firstlight transmission path (16) extends from the first port (12 ₁) of thesplitter/combiner light coupler (12) to the first port (14 ₁) of thefirst transmission path (14 a) of the first optical directional coupler(14). A second light transmission path (20) extends from the second port(12 ₂) of the splitter/combiner light coupler (12) to the first port (14₃) of the second transmission path (14 b) of the first opticaldirectional coupler (14). An optical reflector (22) is coupled to thesecond port (14 ₃, 14 ₄) of one of the first (14 a) and second (14 b)transmission paths of the first optical directional coupler (14). Anoptical absorber (24) is coupled to the second port (14 ₃, 14 ₄) of theother one of the first (14 a) and second (14 b) transmission paths ofthe first optical directional coupler (14). As a result, the lightapplied from the source of light (34) to the light port (12 ₃) of thesplitter/combiner light coupler (12) is modulated by the multiplicativeproduct of the electrical signals applied to the electrical input ports(18 ae, 18 be) of the first (18 a) and second (18 b) electroopticportions. In a particular manifestation of this avatar, the couplingarrangement (28, 38) comprises a second light directional coupler (28)associated with each of the splitter/combiner light coupler (12)s. Eachof the second light directional couplers (28) includes a first port (28₁), a second port (28 ₂), and a third port (28 ₃), for coupling lightapplied to the first port (28 ₁) to the second port (28 ₂), and forcoupling light applied to the second port (28 ₂) to the third port (28₁). The second port (28 ₂) of the second optical directional coupler(28) is coupled to the third port (12 ₃) of the associated one of thesplitter/combiner light coupler (12). As a result, signals applied tothe first port (28 ₁) of the second optical directional coupler (28) arecoupled to the optical modulator (10), and modulated light from theoptical modulator (10) is coupled to the third port (28 ₃) of the secondoptical directional coupler (28) by way of the second port (28 ₂) of thesecond optical directional coupler (28).

A method for receiving signals by way of an array antenna includes thesteps of: (a) generating carrier light; (b) generating a plurality ofelectrical carrier signals, each of which is offset in frequency fromall of the other electrical carrier signals, to thereby produce offsetcarrier signals; (c) applying carrier light to a light splitter, fordividing the carrier light into at least first and second portions,preferably of equal amplitude; (d) transmitting the first portion of thecarrier light to a first port (14 ₃) of a four-port directional coupler(14) by way of a first light path (20); (e) transmitting the secondportion of the carrier light to a cascade of first (18 a) and second (18b) electrooptic modulators; (f) applying received signal from one of theantenna elements (30 a, 30 b, 30N) of the array antenna (30) to one ofthe first (18 a) and second (18 b) electrooptic modulators, formodulating the second portion of the light; (g) coupling one of theplurality of electrical carrier signals to the other one (18 a, 18 b) ofthe electrooptic modulators, for further modulating the second portionof the light; (h) coupling modulated light from the cascade of first (18a) and second (18 b) electrooptic modulators to a second port (14 ₁) ofthe four-port directional coupler (14); (i) reflecting light exitingfrom the third port (14 ₄) of the four-port directional coupler (14), sothat the reflected light re-enters the third port (14 ₄), and flows, atleast in part, to the fourth port (14 ₂) of the four-port directionalcoupler (14), where the reflected light sums with light coupled from atleast one of the first (14 ₃) and second (14 ₁) ports; (j) absorbing anynet light leaving the fourth port (14 ₂), whereby the interaction of thelight reaching the fourth port (14 ₂) variously results in reinforcementand cancellation, which results in mixing of the carrier light, andreceived and electrical carrier signals, to produce carrier light mixedwith the received electrical signals and the one of the offset carriersignals; (k) for each pair of the received signals and electricalcarrier signals, repeating the steps of applying carrier light to alight splitter, transmitting the first portion, transmitting the secondportion, applying received signal, coupling one of the plurality ofelectrical carrier signals, coupling modulated light, reflecting light,and absorbing any net light; (l) combining the resulting carrier lightmixed with the received electrical signals and offset carrier signals,to produce combined signals. The resulting optical signals representingthe received electrical signal at each antenna element can betransmitted over the same optical path, such as an optical fiber path,since the signals are at different optical wavelengths.

What is claimed is:
 1. An optical modulator, comprising: asplitter/combiner light coupler including first, second and third ports,said splitter/combiner light coupler dividing light applied to saidthird port into substantially equal first and second portions at saidfirst and second ports, respectively; a first optical directionalcoupler including first and second coupled transmission paths, saidfirst transmission path of said first optical directional couplerdefining first and second ports, and said second transmission path ofsaid first optical directional coupler defining first and second ports;a first light transmission path including first and second cascadedelectrooptic portions, each of said first and second electroopticportions including an electrical input port, by which the index ofrefraction of said electrooptic portion is responsive to electricalsignals applied to said electrical input port, said first lighttransmission path extending from said first port of saidsplitter/combiner light coupler to said first port of said firsttransmission path of said first optical directional coupler; a secondlight transmission path, said second light transmission path extendingfrom said second port of said splitter/combiner light coupler to saidfirst port of said second transmission path of said first opticaldirectional coupler; an optical reflector coupled to said second port ofone of said first and second transmission paths of said first opticaldirectional coupler; and an optical absorber coupled to said second portof the other one of said first and second transmission paths of saidfirst optical directional coupler, whereby light applied to said thirdport of said splitter/combiner light coupler is modulated by the productof the electrical signals applied to said electrical input ports of saidfirst and second electrooptic portions.
 2. An optical modulatoraccording to claim 1, further comprising a third electrooptic portionincluding an electrical input port, said third electrooptic portionbeing coupled in cascade with said second light transmission path.
 3. Anoptical modulator according to claim 1, further comprising a secondoptical directional coupler including a first port, a second port, and athird port, for coupling light applied to said first port to said secondport, and for coupling light applied to said second port to said thirdport, said second port of said second optical directional coupler beingcoupled to said third port of said splitter/combiner light coupler,whereby signals applied to said first port of said second opticaldirectional coupler are coupled to said optical modulator, and modulatedlight from said optical modulator is coupled to said third port of saidsecond optical directional coupler by way of said second port of saidsecond optical directional coupler.